Middle-aged spread

The Earth is wider at the equator than from pole to pole, mainly because the centrifugal forces generated by its rotation make it bulge outwards. For most of the past 20 years, observations showed that the Earth was getting more rounded at the poles, but in 1998 that trend unexpectedly reversed. Christopher Cox, a geophysicist contracted by NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleague Benjamin Chao discovered the effect in a study of observations from nine satellites (Science, vol 297, p 831).

The new trend implies there has been a transfer of mass from high to low latitudes, but Cox isn't sure why. Melting polar ice and a resulting sea level rise across the world don't explain it. You'd need a block of ice 10 kilometres wide and 5 kilometres high to melt every year to explain ...

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